feat(push-to-gar-docker): enable docker mirror for buildx on self-hosted runners

[ricky-undeadcoders]

Update the push-to-gar-docker to utilize Grafana's internal docker mirror for self-hosted runners.

Key change:

• actions/push-to-gar-docker/action.yaml: Added a conditional expression to set the buildkitd-config parameter to /etc/buildkitd.toml when the runner is self-hosted, and an empty string otherwise. This improves compatibility with self-hosted environments.

This requires https://github.com/grafana/deployment_tools/pull/269388 to be merged first.
Part of https://github.com/grafana/deployment_tools/issues/269366

[iainlane]

From the docs, it seems that the default path is /etc/buildkit/buildkitd.toml. If we use that path in the internal PR you referenced, would there be any client-side config needed?

[ricky-undeadcoders]

From the docs, it seems that the default path is /etc/buildkit/buildkitd.toml. If we use that path in the internal PR you referenced, would there be any client-side config needed?

Interesting, I don't know, but I'll test. I was referencing these docs which suggest creating the config at /etc/buildkitd.toml.

I tested this and it did not work.